An underground city is a series of linked subterranean spaces that may provide a defensive refuge; a place for living, working or shopping; a transit system; mausolea; wine or storage cellars; cisterns or drainage channels; or several of these. The term may also refer to a network of tunnels that connects buildings beneath street level.[citation needed], which may house office blocks, shopping malls, metro stations, theatres, and other attractions. These passages can usually be accessed through the public space of any of the buildings connecting to them, and sometimes have separate entries as well. This latter definition encompasses many modern structures, whereas the former more generally covers tunnel systems from ancient times to the present day.

Underground cities are especially functional in cities with very cold or hot climates, because they permit activities to be comfortably accessible year round without regard to the weather. Underground cities are similar in nature to skyway systems and may include some buildings linked by skyways or above-ground corridors rather than underground.

Possibly the most famous underground cities are Montreal's RÉSO, used by more people than any other locale[citation needed] and the largest underground city network in the world[citation needed], and Toronto's PATH, which according to Guinness World Records, is the largest underground shopping complex in the world with 371,600 square metres (3,999,869 sq ft) of retail space.[1] Japan's underground networks, whereas individually smaller, are the most extensive overall with an estimated 76 underground shopping streets totaling over 900,000 square metres (9,687,519 sq ft) of floor space in 1996,[2] with many expansions since then.

Sydney has a series of underground shopping malls around the underground station Town Hall. The tunnels run south to the George Street cinema district, west under the town hall, and north to Pitt Street Mall through the Queen Victoria Building. The northern branch links Queen Victoria Building with Galleries Victoria, Sydney Central Plaza (which in turn links underground to Westfield CentrePoint and internally above ground to Centrepoint, Imperial Arcade, Skygarden, Glasshouse, and the MLC Centre). The linked centres run for over 3 km (2 mi). In 2005, Westfield corporation submitted a development application to link Sydney Central Plaza underground with 3 other properties on Pitt Street Mall and extend the tunnel network by a further 500 m (1,640 ft) or more.

The small town of Coober Pedy in northern South Australia has numerous underground residences and other facilities. The area was and is extensively mined for opal, and the settlers lived underground to escape the scorching daytime heat, often exceeding 40 °C (104 °F).

Ottawa'sCarleton University has a five kilometer underground tunnel network which connects ten residence buildings with other buildings on its main campus. The city also has an underground concourse at the Place de Ville office complex in the downtown business district, connecting 4 office buildings containing over one million square feet of leasable space, and 2 hotels with 900 rooms combined. There are plans to expand the underground network after the Confederation Line, a rapid transit line featuring three downtown subway stations, is completed. It is estimated there will be 14 buildings with direct indoor connection to the downtown subway portion, or 3 million square feet of office space, 1.5 million square feet of retail, 1400 hotel rooms and the Ottawa Convention Centre.

Saskatoon – On the campus of the University of Saskatchewan a tunnel system connects several of the buildings on campus, this is augmented with overhead walkways that further extend the network.

Winnipeg has a smaller (mainly commercial office) area located underground in the downtown core below Portage and Main. Several of the downtown office towers have subterranean entrances to the complex allowing employees and visitors to bypass the downtown traffic and avoid the cold winter temperatures Winnipeg regularly experiences. The system links with the skyway system known as the Winnipeg Walkway. Also the University of Manitoba has underground tunnels for the students to travel building to building.

Santiago has some elements of an underground city in its "Metro" subway system. While all stations have a small mezzanine level above the tracks for ticket purchase, some key stations have extensive areas of shops and kiosks in addition. Some stations even have an additional mall-like level between the street and the mezzanine levels.

Beijing built an extensive tunnel network called the Underground City (Chinese: 地下城; pinyin: Dìxià Chéng) during the Sino-Soviet conflict, supposedly covering 85 km², falling into disuse in the 1970s. It has been recently[when?] opened to the public and tourists. There are rumors it was used by the army during the Tiananmen incident. As of 1 July 2009, all "official" remaining entrances appear to be closed.

Harbin has a number of large, multi-level underground shopping areas, originally built for air defence. The largest is at the roundabout intersection of Xida Zhi street and Hongjun street where three levels of markets following streets from four directions meet under the giant snowflake atrium.

Qingdao has two small underground shopping areas, one at the head of the Zhanqiao (pier) and one west of the Qingdao guest house.

Shanghai has a few underground networks, most notably at the People's Square metro station, wherein the line 2 station has a second mezzanine full of shops and line 1 is connected to a large underground shopping gallery at its south end. Shanghai Science Museum stop on line 2 has a large underground shopping area, known for its imitation goods. Huangpi Road South and Xujiahui stations are directly connected to shopping centers, and the Lujiazui station is connected to the Bank of China tower.

Paris: The Forum des Halles is a partially underground multiple storey commercial and shopping center, opened in 1979. The building is connected to the underground station Chatelet-Les-Halles, central hub of Paris's express metro system, the RER. There was more than 300 km (186 mi) of a network of catacombs, started as limestone quarries and later for burials, used as safe passageways in times of revolution.

Naours: 33 metres (108 ft) below the Picardy village of Naours (between Doullens and Amiens), the 28 galleries (2,000 metres (6,562 ft) long) of an ancient limestone quarry (exploited since the 3rd century c.e.) have long been used as shelter by the population seeking refuge from invaders. Occupied by the Triple Entente forces during WWI, and then used as headquarters by the German army during the WWII occupation of France, the galleries are now open to visitors.

Berlin: Several buildings on the east side of Friedrichstraße, from Quartier 205 (Friedrichstraße 70) northwards up to Galerie Lafayette are connected to each other. Businesses are on both sides of the underground street so that it appears to be inside a building all the time, even when it crosses Taubenstraße underground.

Frankfurt am Main: Underground shopping malls, called 'B-Ebene' in Frankfurt, of considerable size are found at Hauptbahnhof (central station) and Hauptwache.

Hamburg: The Jungfernstieg and Rathaus subway stations and several entrances are connected by pedestrian tunnels. Some of them contain shops. There is a direct access to the Europapassage shopping mall.

Stuttgart: There is a large underground shopping mall (Klettpassage) connecting the Hauptbahnhof (central station) with the main shopping street, the Königstraße.

In Thessaloniki, Hellenistic, Roman, Byzantine and Ottoman monuments coexist[3] that bear witness to the city history and its significance. Several of the underground secrets of Thessaloniki have been discovered and emerged, while restoration work is under process. Although the final result is impressive with underground monuments such as the Catacombs of St. John and the underground museum of the Agora, they do not comprise highlights for the city and access is still very limited or not permitted even to the locals by the Archaeological Department of the Thessaloniki underground Metro project.

Osaka has enormous underground networks in the Umeda, Namba, and Shinsaibashi districts, in which Umeda alone includes over 1,200 retail stores and restaurants, as well as subway and intercity rail stations.

Tokyo's subway lines are owned by Tokyo Metro (9 lines) and Toei (4 lines), mostly within the 23 special wards. Near the center of the city, connecting passages between stations that span a few blocks exist for commuters. Stations such as Shinjuku and Shibuya have underground shopping malls. Tokyo's network of tunnels is seen as one of the many megalopolis features of the city.

In general, many large railway stations house underground hallways featuring shops, restaurants, banks and money exchange offices. A striking example of such stations would be the main hallway of the Amsterdam central station, which connects to the city's metro system, although due to renovation and re-building it is temporarily (2012–2015) no longer possible to walk from the subway to the train station without going outside.

Maastricht: Originally a casemate, the kazematten of Maastricht form a 14 km (9 mi) long network of tunnels underneath the western part of the city.[4] This tunnel network has mainly been used for military purposes. The main construction period of these tunnels lasted from 1575 to 1825.[4] The newest sections of the tunnel network was dug as late as the middle 20th century, built in the Cold War as a shelter for citizens in the event of a nuclear strike on the city.[5] Another, far more extensive, system of tunnels with a length over 200 km (124 mi) and 20,000 individual corridors lies just west of Maastricht; this is referred to as the Caves of Maastricht (Dutch: Grotten van Maastricht).[6] These man-made 'caves' were used as Marl quarries from the 13th century onwards.[7] In World War II, these caves were used to hide large quantities of paintings from the Germans, even including the Nachtwacht.[8] In 1944, construction started on a large public shelter that could have housed 45,000 persons in these caves. The project never saw its completion due to the liberation of Maastricht in the fall of the same year.[9]

During World War II, "German soldiers have constructed extensive "underground cities" in the swampy Russian plains close to the battlefront along the Volkhov River which runs from Lake Ilmen to Lake Ladoga south and east of Leningrad. Birch trees abound here and provide materials not only for the buildings but also for hundreds of miles of corduroy motor roads across the swamp. Owing to the nature of the ground, the huts must be constructed on slopes where good drainage is possible. Corduroy roads and walks connecting the buildings are necessary because of the soft, slippery clay soil. It is evident that considerable care has been taken to make these dwellings as attractive as possible both inside and out. Six or eight men occupy each hut and there are also underground stables and storage places for coal and supplies."[11]

In Barcelona, there's an abandoned underground mall near Plaça de Catalunya called Avinguda de la Llum, closed since 1990, which had originally been part of a more ambitious project to build an underground city under the centre of Barcelona. Also, some Metro stations or connecting lines in the same station are connected by underground passages over a block in length.

Legend has that the many caves and tunnels under the old part of Toledo were connected and were used by occultists.

By walking through Stockholm subway stations and indoor shopping malls it is possible to walk indoor through the central business district, partly underground, from Arsenalsgatan subway entrance (Kungsträdgården station) to Kungsbron (north entrance to Cityterminalen bus terminal), covering a distance of between 1 and 2 kilometres.

Cappadocia contains several historical underground cities carved out of unusual geological formations formed via the eruptions of ancient volcanoes. The cities were initially inhabited by the Hittites, then later by early Christians as hiding places (see [1]). They are now archeological and tourist sites but are not generally occupied (see Kaymaklı Underground City and Derinkuyu). The latest large underground city was discovered in 2007 in Gaziemir, Güzelyurt. It was a stopover on the Silk Road, allowing travelers and their camels to rest in safety underground, in a 'fortress' equivalent to a modern hotel.

A typical view from inside the underground city in Derinkuyu, one of the largest underground complexes in Cappadocia. There are few artifacts left from the original builders, mainly just large rocks that were used to block the passage for intruders. Most of the "cities" are corridors, but some places there are rooms large enough to live in, and some rooms that have the cross-like shape of a church, which is probably exactly what they were.

Istanbul boasts the Roman cisterns, built 2000 years ago for water storage but now a tourist attraction.

Corsham, Wiltshire is the location of the Central Government War Headquarters, code name 'Burlington'. Built in the late 1950s in response to the increasing threat of nuclear warfare during the Cold War, the 35 acres (1,524,600 sq ft) subterranean site was designed to be the main emergency government war headquarters of the UK outside of London and safely house up to 4,000 central government personnel in the event of a nuclear strike.

Dover contains a series of interconnecting tunnels, that honeycomb both sides of the Dover Valley, carved into the chalk cliffs. These date from Ancient times at Dover Castle through to Napoleonic, Second and Cold War installations. The Dover Castle complex is the larger, going at least six levels deep and includes a hospital, troops quarters, offices and storage and channel view points. The southern tunnels are mixed between Napoleonic War-era defences (see Dover Western Heights) and Second World War-era defences, with some seafront air raid shelters still used for shop storage today. Many have fallen into disrepair and are now closed to the public, but many are still open.

There are extensive underground constructions across Britain, built as air-raid shelters during World War II.

Edinburgh's old town has extensive rooms, tunnels and chambers beneath some areas; of particular note are the Edinburgh Vaults, where overcrowding led people to construct elaborately interconnected buildings in the vaults of the city's South Bridge.

In Southport, Merseyside, Nevill Street has the remainders of an underground shopping street, which can now only be accessed from the cellars of buildings on the current street, which was raised by one storey from the original level. One end of the underground street ended at the Marine Lake, close to the pier entrance. [2];

in Liverpool, Merseyside, the Williamson's tunnels included the site of an 'underground house' complete with windows (concealed by work for public opening) and an extant and partially excavated 'banqueting hall'.

Atlanta, Georgia's “Underground” represents the original surface level of downtown Atlanta; the present streets are raised roadways (viaducts) built in the 1920s. The shopping center Underground Atlanta, taking advantage of the former street-level storefronts, covers six city blocks and includes retail shopping and restaurants. It was begun in 1968 and re-opened in 1989 after a financially forced closure.[14]

Boise, Idaho's Capital Mall Complex in downtown Boise consists of a large system of networked tunnels that connect all the state buildings. The tunnels have walkways and vehicle passageways. The underground area boasts a geothermal power plant, a banking system, extensive dining areas, parking, a dedicated mail room for the Capital Mall Complex and a fallout shelter. The main hall is decorated with art from local artists that was collected over a thirty-year time frame. This collection is not often seen by the public as access is limited. Recent remodeling of the Idaho State Capitol Building has added new underground wings that are linked to the Capital Mall. These new wings have offices, meeting rooms and records storage areas and were designed to support the Idaho legislature when it is in session. Local folklore circulates stories that the Capital Mall Complex may be connected to the original Chinese underground tunnels that were used so Chinese immigrants could move goods to buildings, smuggle opium, and travel after curfew. However, there is little evidence these rumors are true.

Houston, Texas: The seven-mile (11 km (7 mi)) Houston tunnel system is set about twenty feet below Houston's downtown street system and is composed of underground passageways which, with above-ground skywalks, link office towers to hotels, banks, corporate and government offices, restaurants, retail stores, and the Houston Theater District. Only one building, Wells Fargo Plaza, offers direct access from the street to the Tunnel; otherwise, you must enter the Tunnel from street-level stairs, escalators, or elevators inside a building connected to it.

Oklahoma City, Oklahoma: The Oklahoma City Underground, formerly the Oklahoma City Conncourse, named after its founder, Jack Conn, is an underground tunnel system connecting nearly all the downtown buildings in a 20-square-block area. The OKC Underground is one of the most extensive all-enclosed pedestrian systems in the U.S., extending three-quarters of a mile and connecting over 30 downtown buildings by tunnel or skyway. The original tunnel link was built in 1931 and the system was extended in the 1970s. Offices, shops, and restaurants line the OKC Underground system. It underwent a $2 million renovation in 2006–2007. Upon completion of the renovation, the Conncourse was renamed the Underground.[17][18]

Richmond, Virginia: There are a series of connected tunnels between state government buildings in the city of Richmond. Certain passageways are locked off but a good portion of the tunnels are accessible from buildings. The purpose of the tunnels is not generally known; the two most common explanations are that they were built to allow people to move between buildings in inclement weather or that they were built as part of an emergency evacuation plan.

Rochester, Minnesota: The Mayo Clinic's buildings in the downtown area are interconnected with tunnels and skyways. Other businesses are along the corridors, including a number of hotels that often house clinic patients. It is often called a subway, although there are no underground rails in the city.

SubTropolis: A 55,000,000 square foot (5,060,000 m³), 1,100-acre (4.5 km2) underground business complex along the Missouri River in Kansas City, Missouri. The space was originally part of the Bethany Falls limestone mine and was later repurposed for use as a commercial underground storage facility.

Vicksburg, Mississippi During the Siege of Vicksburg in 1863, Union gunboats lobbed over 22,000 shells into the town and army artillery fire was even heavier. As the barrages continued, suitable housing in Vicksburg was reduced to a minimum. A ridge, located between the main town and the rebel defense line, provided a diverse citizenry with lodging for the duration. Over 500 caves were dug into the yellow clay hills of Vicksburg. Whether houses were structurally sound or not, it was deemed safer to occupy these dugouts. People did their best to make them comfortable, with rugs, furniture, and pictures. They tried to time their movements and foraging with the rhythm of the cannonade, sometimes unsuccessfully. Because of these dugouts or caves, the Union soldiers gave the town the nickname of "Prairie Dog Village." Despite the ferocity of the Union fire against the town, fewer than a dozen civilians were known to have been killed during the entire siege.

Walt Disney World, Florida (southwest of Orlando, Florida) has a network of utility tunnels used by its employees (cast members) for transportation between venues, rest areas, staff preparation, and first aid. The main system is under the Magic Kingdom theme park. Other tunnels lie under Future World at Epcot.